This invention relates to an ultrasonic probe with the use of a polymeric piezoelectric member as a vibrator.
In the prior art, as a linear array type ultrasonic probe used, for example, in a linear electron scanning system, one employed an array having a ceramic piezoelectric member such as lead titanate, lead titanate-zirconate, etc. cut into rectangular strips. (See for example, J. F. Havlice and J. C. Tazer, "Medical Ultrasonic Imaging: An Overview of Principles and Instrumentation", Proc. IEEE Vol. 67, p. 620 (1979) and A. Fukumoto, "The Application of Piezoelectric Ceramics in Diagnostic Ultrasound Transducer", Ferroelectrics, Vol. 40, p. 217 (1982)). However, such a ceramic piezoelectric member has rigid and brittle properties, is prone to generation of defects or fractures during dividing by cutting, and difficulties are encountered in precise formation of a number of electrodes shaped in rectangular strips, whereby, problems arise from the cost aspect as well.
In contrast, fluorine containing polymers such as polyvinylidene fluoride (hereinafter abbreviated as PVF.sub.2), polyvinylidene fluoride-trifluoroethylene copolymer (hereinafter abbreviated as PVF.sub.2.TrFE) and other polar synthetic polymers are known to exhibit piezoelectric property and pyroelectric property by being subjected to a polarizing treatment under high temperature and high electrical field. (See, for example, Y. Higashihata, J. Sako and T. Yagi, "Piezoelectricity of PVF.sub.2.TrFE", Ferroelectrics, Vol. 32, pp. 85-92, (1981)). Also, development of the ultrasonic probe utilizing thickness vibration of the aforesaid polymeric piezoelectric member has been actively done in recent years. Such a polymeric piezoelectric member has an inherent acoustic impedance which is approximate to that of water or a living body and also small in modulus, and therefore, when a polymeric piezoelectric member is applied for a linear array type ultrasonic probe, as different from the example of a ceramic piezoelectric member, it is said that the polymeric piezoelectric member itself is not necessarily required to be cut and separated into rectangular strips and is required to be separated only as an electrode.
However, the dielectric constant of a polymeric piezoelectric member is markedly smaller as compared with a ceramic piezoelectric member, namely in the order of generally about 10, and also due to the small area of the driving element of the linear array type ultrasonic probe the, electrical impedance becomes markedly higher, whereby electrical matching with a 50 .OMEGA. system power source (sending and receiving circuits) is ordinarily poor which results in a marked loss and lowering of the ultrasonic wave.
For such reasons as mentioned above the, usefulness of a so-called laminated piezoelectric ultrasonic probe, in which a plurality of polymeric piezoelectric members are laminated appropriately so that the polarized axis directions may be opposed to each other, has been investigated (for example, Japanese Provisional Patent Publications No. 151893/1980 and No. 47199/1981). Such a laminated polymeric piezoelectric member is laminated by adhering two sheets of polymeric piezoelectric members having, for example, a film thickness t under the state with an electrode interposed therebetween so that the polarized axis directions may be opposed to each other. On one surface of such a laminated polymeric piezoelectric member is provided an acoustic reflective plate (.lambda./4 plate), connecting the piezoelectric member to the electrode of the same direction as the polarized axis direction. Upon applying voltage pulses, etc. thereon, excitation of an ultrasonic wave conforming to the basic mode of: EQU .lambda./4=2t (.lambda.=8t)
becomes possible. That is, as compared with the case of constituting the polymeric piezoelectric member of one sheet with a film thickness of 2t, the electrical capacity of the polymeric piezoelectric member becomes 4-fold resulting in an electrical impedance of 1/4.
However in an ultrasonic probe with such a structure, during lamination of the polymeric piezoelectric, members, electrodes shaped in rectangular strips can only be accurately made in conformity to each other with difficulty, and deviation in position is liable to occur between the upper and lower electrodes. With occurence of such a deviation in position, not only does the electrical impedance of the polymeric piezoelectric member previously designed fail to exhibit its initial characteristics, but also the output ultrasonic wave beomces non-uniform due to non-uniformity of the thickness vibration mode, etc. and simultaneously there occurs generation of acoustic-electrical coupling or cross-talk, whereby sensitivity may be lowered or the band region narrowed, even resulting in generation of a short circuit between the driving elements. This problem becomes more marked as the number of the polymeric piezoelectric members is increased.
On the other hand, the electrodes shaped in rectangular strips are generally of a miniature size, and can be formed by vapor deposition or patterning of a metal film according to the vapor deposition method, the sputtering method, etc. However, if the film thickness of the metal film constituting the electrodes is thin, the electrical resistance becomes high to cause loss of the voltage driving pulses. Also, during lamination of the polymeric piezoelectric members, when lamination is effected by folding one continuous polymeric piezoelectric material, there is the danger that electrodes shaped in rectangular strips may be broken.
Also, since the aforesaid electrodes shaped in rectangular strips form an inherent electrode pattern on a polymeric piezoelectric member, it is very cumbersome to take out the lead wires from the electrodes. For example, in taking out lead wires from the electrodes shaped in rectangular strips which have been obtained by working the electrode imparted on the whole surface by vacuum vapor deposition on a polymeric piezoelectric member by etching into rectangular strips, it is impossible to take out lead wires by direct soldering of lead wires because of softening of the polymeric piezoelectric member (in the case of PVF.sub.2, a softening point of about 170.degree. C.) or depolarization. For this reason, for example, there is employed the method wherein the lead wires are taken out while securing the lead wires with the use of a so-called electroconductive adhesive or an electroconductive paint in which electroconductive powder such as silver powder is mixed into an adhesive. However, in such a method, there are involved the problems such that short circuit of electrodes shaped in rectangular strips through the electroconductive adhesive or the electroconductive paint or peel-off of the lead wire secured portion will readily occur, and also that changes with lapse of time occur such as the lowering in securing force and elevation in resistance value.
Since the dielectric constant of the polymeric piezoelectric member is generally small in the order of 10 to some hundreds and is about several hundredth to several tenth as compared with a ceramic piezoelectric member with several thousands or so, in case of the array type ultrasonic probe having a small driving surface per one element, electrical impedance becomes markedly higher. Thus, there are problems that electrical matching with an usual 50 .OMEGA. driving circuit or a receiving circuit is difficult whereby the charateristics of the ultrasonic probe will be deteriorated.
Further, since the polymeric piezoelectric member has a high electrical impedance as mentioned above, when it is used by connecting a coaxial cable of a 50 .OMEGA. or 75 .OMEGA. system, a length of a coating layer on a core wire of a cable to be connected and a length of a ground wire to be taken-out become a problem, and in certain circumstances, there occurs a problem of a so-called cross-talk phenomenon where other elements are to be driven.